Wireless internet and cell phone services operate primarily in wireless frequency bands allocated by the FCC for that purpose. These licensed bands regulate usage to minimize signal congestion and prevent impaired functioning due to destructive interference between incompatible network and signal formats. While the use of licensed bands circumvents the interference vulnerability of existing wireless networks, it has resulted in a scarcity of usable wireless spectrum. Certain other wireless spectrum is designated as unlicensed, not requiring a license from the FCC. For example, Wi-Fri networks and some variations of the Long-Term Evolution (LTE) operate in unlicensed spectrum (e.g., in the unlicensed 5 GHz band). In the face of diminishing licensed band availability and increasing costs of obtaining spectrum licenses, a demand exists for wireless services to utilize unlicensed spectrum. However, in order to be commercially deployable, systems may need the capability to mitigate the effects of a broad range of radio-frequency (RF) interference from other users in those unlicensed bands. A similar need for interference mitigation is also arising in licensed bands, as signal crowding continues to increase in nearly all bands allocated for wireless services.
Wireless networks equipped with antenna arrays, e.g., equipped with multiple antenna elements at each network node, such as in multiple-input-multiple-output (MIMO) systems (e.g. Wi-Fi) and emerging massive MIMO systems, have the potential to exploit the spatial diversity afforded by such antenna arrays to achieve enhanced functioning in interference, noise, multipath, and other common radio signal propagation impairments, by using the arrays to dynamically shape directional response patterns (e.g., peaks and nulls) to maximize signal quality while reducing interference power. However, this potential, especially for interference mitigation, has not been the focus of these array-based systems, which limit their use of spatial diversity to generating multi-stream MIMO links based on standard spatial multiplexing methods, and/or to simple beam-pointing to increase link performance. None of these methods explicitly mitigate interference, and in fact they are highly vulnerable to it. Accordingly, array-based processing represents a largely untapped enabling technology for migrating existing and future wireless WAN systems into unlicensed bands.
Inherent to all prominent and/or practical wireless WAN standards (e.g., 3G, 4G, 5G, and proprietary variants) is a need for remote nodes (RNs) to first discover a network and achieve frequency- and time-synchronization to that network. This is an essential prerequisite to any form of substantive network (e.g., internet) access activity. For example, a RN may identify and synchronize to an intended base station node (BN). For reliable operation in unlicensed and increasingly crowded licensed bands, these essential wireless network discovery and synchronization functions will require interference mitigation, methods for which are described herein.